Abstract:

There is provided a variable device circuit according to the present
invention, including: a substrate; at least one movable switch device
formed on a first principal surface of the substrate; at least one fixed
capacitor device formed on the first principal surface of the substrate;
at least one variable capacitor device formed on the first principal
surface of the substrate; at least one variable inductor device formed on
the first principal surface of the substrate; and wiring lines for
electrically connecting the devices to one another, the wiring lines
being formed on the first principal surface of the substrate; wherein
electrical connections among the devices can be selected by operation of
the movable switch device, whereby achieving stable, low-loss circuit
characteristics with lower manufacturing cost.

Claims:

1. A variable device circuit comprising:a substrate;at least one movable
switch device formed on a first principal surface of the substrate;at
least one fixed capacitor device formed on the first principal surface of
the substrate;at least one variable capacitor device formed on the first
principal surface of the substrate;at least one variable inductor device
formed on the first principal surface of the substrate; andwiring lines
for electrically connecting the devices to one another, the wiring lines
being formed on the first principal surface of the substrate;wherein
electrical connections among the devices can be selected by operation of
the movable switch device.

2. The variable device circuit according to claim 1, wherein the fixed
capacitor device includes a lower electrode on the substrate side, an
upper electrode opposite to the lower electrode, a dielectric layer and
an air layer, both of which are interposed between the lower electrode
and the upper electrode, andthe air layer is thicker than the dielectric
layer.

3. The variable device circuit according to claim 1, wherein the movable
switch device, the variable capacitor device and the variable inductor
device include a lower electrode on the substrate side, an upper
electrode opposite to the lower electrode, and an air layer which is
interposed between the lower electrode and the upper electrode, anda
dielectric layer is formed on the lower electrode.

4. The variable device circuit according to claim 1, wherein a lower
electrode of each device is formed of a first metal layer having a higher
resistance and a second metal layer having a lower resistance, anda bias
line for driving the device is formed of the first metal layer and a
dielectric layer covering the first metal layer.

5. The variable device circuit according to claim 1, wherein the air
layers, each interposed between the lower electrode and the upper
electrode of each device, are substantially equal in thickness to one
another.

6. The variable device circuit according to claim 1, wherein a coplanar
transmission line including a signal line and ground lines located on
both sides of the signal line is provided on the first principal surface
of the substrate, andat least one of the movable switch device, the fixed
capacitor device, the variable capacitor device and the variable inductor
device is inserted in the signal line.

7. The variable device circuit according to claim 6, wherein a bias line
for driving the device intersects the ground line with an interposed air
layer.

8. The variable device circuit according to claim 6, wherein a second
ground line, which is provided for electrically connecting the ground
lines located on both sides of the signal line to each other, intersects
the signal line with an interposed air layer.

9. The variable device circuit according to claim 7, wherein both the air
layer interposed between the lower electrode and the upper electrode of
each device and the air layer interposed between the bias line and the
ground line have substantially the same thickness.

10. The variable device circuit according to claim 8, wherein both the air
layer interposed between the lower electrode and the upper electrode of
each device and the air layer interposed between the second ground line
and the signal line have substantially the same thickness.

11. A method for manufacturing a variable device circuit, including steps
of:forming a first conductive layer with a predetermined pattern on a
first principal surface of a substrate;forming a dielectric layer with a
predetermined pattern on the first principal surface of the substrate and
the first conductive layer;forming a sacrificial layer with a
predetermined pattern, which is thicker than the dielectric layer, on the
first principal surface of the substrate, the first conductive layer and
the dielectric layer;forming a second conductive layer with a
predetermined pattern on the first principal surface of the substrate,
the first conductive layer, the dielectric layer and the sacrificial
layer; andforming an air layer between the first conductive layer and the
second conductive layer by removing the sacrificial layer;wherein at
least two out of a movable switch device, a fixed capacitor device, a
variable capacitor device and a variable inductor device are formed
concurrently.

12. The method according to claim 11, wherein the first conductive layer
includes a first metal layer having a higher resistance and a second
metal layer having a lower resistance, andthe method further including a
step of forming a bias line for driving the device by removing the second
metal layer of the first conductive layer which is formed in a
predetermined pattern, followed by covering the first metal layer with a
dielectric layer.

13. The method according to claim 11, wherein the first conductive layer
constitutes a lower electrode for at least two devices formed
concurrently, and the second conductive layer constitutes an upper
electrode of at least two devices formed concurrently.

14. The method according to claim 13, wherein air layers interposed
between the lower electrode and the upper electrode of at least two
devices formed concurrently have substantially the same thickness.

15. The method according to claim 11, further including a step of forming
a coplanar transmission line, which includes a signal line and ground
lines located on both sides of the signal line, by using the first
conductive layer and the second conductive layer.

16. The method according to claim 15, wherein at least two devices formed
concurrently are inserted in the signal line.

17. The method according to claim 15, further including a step of forming
the ground line so as to intersect a bias line for driving the device
with an interposed air layer which is obtained by removal of the
sacrificial layer.

18. The method according to claim 15, further including steps of:forming a
second ground line for electrically connecting the ground lines located
on both sides of the signal line to each other; andforming the signal
line so as to intersect second ground line with an interposed air layer
which is obtained by removal of the sacrificial layer.

19. The method according to claim 17, wherein both the air layer
interposed between the lower electrode and the upper electrode of each
device and the air layer interposed between the bias line and the ground
line have substantially the same thickness.

20. The method according to claim 18, wherein both the air layer
interposed between the lower electrode and the upper electrode of each
device and the air layer interposed between the second ground line and
the signal line have substantially the same thickness.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to variable device circuits and method
for manufacturing the same, which can be formed by applying
micromachining techniques or the like and which is suitable for
radio-frequency variable components to be used in radio-frequency bands
such as microwaves, semi-millimeter waves and millimeter waves.

[0003]2. Description of the Related Art

[0004]Recently, radio-frequency devices using micromachining techniques,
so called RFMEMS (Radio Frequency Micro-Electro-Mechanical-Systems)
devices, have been drawing attention. With this technique, high-aspect
three-dimensional structures, hollow structures, variable structures and
the like can be easily manufactured, thereby expectably allowing
radio-frequency devices and circuits to be fabricated on low-priced
silicon substrates with low cost, low loss, high isolation, and
high-performance.

[0005]Also recently, RFMEMS radio-frequency variable circuits made up of
RFMEMS active devices and RFMEMS passive devices, based on mechanical
switches, have been in the spotlight as a key technology to
next-generation radio-frequency modules that are required to operate on
plural frequency bands. Further, RFMEMS radio-frequency variable
circuits, when integrated in combination with existing CMOS
high-frequency circuits, are expected to realize radio-frequency modules
with higher functions, lower cost and lower size.

[0006]In addition, research and development of radio-frequency variable
devices and circuits using micromachining techniques have been being
advanced. For instance, Non-Patent Document 1 shown below, discloses a
radio-frequency variable capacitor (hereinafter, referred to as first
prior art) formed by using micromachining techniques.

[0007]The radio-frequency variable capacitor according to the first prior
art is configured of a coplanar transmission line formed on a substrate,
a bridge structure formed above a signal line of the coplanar
transmission line, and a movable beam having an electrode connected from
above the bridge through a pole.

[0008]Application of a driving voltage between the movable beam and the
substrate causes the movable beam to be pulled toward the substrate by
electrostatic force, so that the bridge which is connected thereto
through the pole and provided on the coplanar transmission line is
deformed. This deformation leads to a change in distance between the
signal line of the coplanar transmission line and the bridge crossing
thereover, which in turn results in a change in capacitance between the
signal line of the coplanar transmission line and the bridge, hence,
thereby acting as a variable capacitor. The change of the capacitance can
be adjusted by the driving voltage applied to the movable beam.

[0009]Patent Document 1 shown below also discloses a radio-frequency
variable device formed by using micromachining techniques (hereinafter,
referred to as second prior art).

[0010]The radio-frequency variable device according to the second prior
art is configured of a variable capacitor having a drive mechanism formed
on the top face of the substrate using surface micromachining techniques,
a fixed capacitor implemented by a pair of plural electrodes formed on
the rear face of the substrate using bulk micromachining techniques, a
switch formed on the top face of the substrate using surface
micromachining techniques, and wiring lines for electrically connecting
the switch, the variable circuit and the fixed capacitor to one another.

[0011]The fixed capacitor composed of a pair of plural electrodes is
provided in plurality, and a fixed capacitor having a desired capacitance
is selected by using the switch.

[0012]In the variable capacitor, a movable membrane and an upper electrode
are provided above the wiring lines coupling to a lower electrode, in
which application of a driving voltage between the lower electrode and
the upper electrode causes the membrane to be deformed by electrostatic
force, which in turn results in a change in distance between the upper
electrode and the lower electrode so that the capacitance can change,
thereby acting as a variable capacitor.

[0013]A fixed capacitor selected by the switch is electrically
communicated with the variable capacitor, and the operating frequency of
the device can be tuned stepwise by the selected fixed capacitor and
moreover fine tuned by the variable capacitor.

[0015]The radio-frequency variable capacitor according to the first prior
art is configured of the bridge and the movable beam having the electrode
above a coplanar transmission line, in which the bridge is connected to
the movable beam via through the pole. In this case, a complicated drive
mechanism is required, and the variable capacitor is not easy to
manufacture.

[0016]Also, since the radio-frequency variable capacitor has insecure
factors in terms of mechanical strength, there is a need for preparing a
large-sized movable beam to give enough electrostatic force to the
movable beam. This would lead to increases in size of the device or the
like, entailing a problem that integration of switches, fixed capacitors,
and variable inductors becomes difficult in manufacturing.

[0017]Further, although the relative variability ratio of capacitance is
large, the area of the bridge as well as the distance between the bridge
and the signal line of the coplanar transmission line are limited in
terms of the device structure and its manufacture, so that the resultant
capacitance is also limited. As a result, it is difficult to form the
bridge and the movable beam above a thick-film coplanar transmission line
because of a large step gap. This leads to an issue that conductor loss
of the coplanar transmission line is hard to reduce.

[0018]On the other hand, in the case of the radio-frequency variable
device according to the second prior art, the switch and the variable
capacitor are fabricated on the top face of the substrate by using
surface micromachining, and the fixed capacitor is fabricated on the rear
face of the substrate by bulk micromachining. This would leads to a
complexity in the fabrication process.

[0019]Also, because of increases in insertion loss due to through-wiring
that connect the top face with the rear face of the substrate, a deep
digging process of the substrate is indispensable in the fabrication
process of the fixed capacitor. This makes a weak point of the
radio-frequency variable capacitor in terms of its manufacturing cost.

[0020]Furthermore, in a comb-tooth structure making up the fixed
capacitor, there is a high possibility of short circuits between
electrodes due to metallization. It is also difficult to provide a lid
structure, which is necessary to obtain stable electrical
characteristics, on the rear face on which the fixed capacitor is formed.
Still more, because of the structure that has difficulty in forming a
dielectric film, there is a limitation of capacitance.

[0021]Moreover, for the variable capacitor, there is a high possibility of
short circuits between the upper electrode (movable electrode) and the
lower electrode due to a high step gap, giving rise to a problem in
operational reliability of the fixed capacitor and the variable
capacitor.

SUMMARY OF THE INVENTION

[0022]It is an object of the present invention to provide variable device
circuits and method for manufacturing the same, which has stable,
low-loss circuit characteristics with lower manufacturing cost by
concurrent formation of variable devices and fixed passive devices, each
having nearly the same device structure, on one surface of a substrate.

[0023]In order to achieve the above object, according to an aspect of the
present invention, there is provided a variable device circuit including:

[0024]a substrate;

[0025]at least one movable switch device formed on a first principal
surface of the substrate;

[0026]at least one fixed capacitor device formed on the first principal
surface of the substrate;

[0027]at least one variable capacitor device formed on the first principal
surface of the substrate;

[0028]at least one variable inductor device formed on the first principal
surface of the substrate; and

[0029]wiring lines for electrically connecting the devices to one another,
the wiring lines being formed on the first principal surface of the
substrate;

[0030]wherein electrical connections among the devices can be selected by
operation of the movable switch device.

[0031]It is preferable that the fixed capacitor device includes a lower
electrode on the substrate side, an upper electrode opposite to the lower
electrode, a dielectric layer and an air layer, both of which are
interposed between the lower electrode and the upper electrode, and the
air layer is thicker than the dielectric layer.

[0032]It is preferable that the movable switch device, the variable
capacitor device and the variable inductor device include a lower
electrode on the substrate side, an upper electrode opposite to the lower
electrode, and an air layer which is interposed between the lower
electrode and the upper electrode, and a dielectric layer is formed on
the lower electrode.

[0033]It is preferable that a lower electrode of each device is formed of
a first metal layer having a higher resistance and a second metal layer
having a lower resistance, and

[0034]a bias line for driving the device is formed of the first metal
layer and a dielectric layer covering the first metal layer.

[0035]It is preferable that the air layers, each interposed between the
lower electrode and the upper electrode of each device, are substantially
equal in thickness to one another.

[0036]It is preferable that a coplanar transmission line including a
signal line and ground lines located on both sides of the signal line is
provided on the first principal surface of the substrate, and

[0037]at least one of the movable switch device, the fixed capacitor
device, the variable capacitor device and the variable inductor device is
inserted in the signal line.

[0038]It is preferable that a bias line for driving the device intersects
the ground line with an interposed air layer.

[0039]It is preferable that a second ground line, which is provided for
electrically connecting the ground lines located on both sides of the
signal line to each other, intersects the signal line with an interposed
air layer.

[0040]It is preferable that both the air layer interposed between the
lower electrode and the upper electrode of each device and the air layer
interposed between the bias line and the ground line have substantially
the same thickness.

[0041]It is preferable that both the air layer interposed between the
lower electrode and the upper electrode of each device and the air layer
interposed between the second ground line and the signal line have
substantially the same thickness.

[0042]According to another aspect of the present invention, there is also
provided a method for manufacturing a variable device circuit, including
steps of:

[0043]forming a first conductive layer with a predetermined pattern on a
first principal surface of a substrate;

[0044]forming a dielectric layer with a predetermined pattern on the first
principal surface of the substrate and the first conductive layer;

[0045]forming a sacrificial layer with a predetermined pattern, which is
thicker than the dielectric layer, on the first principal surface of the
substrate, the first conductive layer and the dielectric layer;

[0046]forming a second conductive layer with a predetermined pattern on
the first principal surface of the substrate, the first conductive layer,
the dielectric layer and the sacrificial layer; and

[0047]forming an air layer between the first conductive layer and the
second conductive layer by removing the sacrificial layer;

[0048]wherein at least two out of a movable switch device, a fixed
capacitor device, a variable capacitor device and a variable inductor
device are formed concurrently.

[0049]It is preferable that the first conductive layer includes a first
metal layer having a higher resistance and a second metal layer having a
lower resistance, and the method further including a step of forming a
bias line for driving the device by removing the second metal layer of
the first conductive layer which is formed in a predetermined pattern,
followed by covering the first metal layer with a dielectric layer.

[0050]It is preferable that the first conductive layer constitutes a lower
electrode for at least two devices formed concurrently, and the second
conductive layer constitutes an upper electrode of at least two devices
formed concurrently.

[0051]It is preferable that air layers interposed between the lower
electrode and the upper electrode of at least two devices formed
concurrently have substantially the same thickness.

[0052]It is preferable that the method further includes a step of forming
a coplanar transmission line, which includes a signal line and ground
lines located on both sides of the signal line, by using the first
conductive layer and the second conductive layer.

[0053]It is preferable that at least two devices formed concurrently are
inserted in the signal line.

[0054]It is preferable that the method further includes a step of forming
the ground line so as to intersect a bias line for driving the device
with an interposed air layer which is obtained by removal of the
sacrificial layer.

[0055]It is preferable that the method further includes steps of:

[0056]forming a second ground line for electrically connecting the ground
lines located on both sides of the signal line to each other; and

[0057]forming the signal line so as to intersect second ground line with
an interposed air layer which is obtained by removal of the sacrificial
layer.

[0058]It is preferable that both the air layer interposed between the
lower electrode and the upper electrode of each device and the air layer
interposed between the bias line and the ground line have substantially
the same thickness.

[0059]It is preferable that both the air layer interposed between the
lower electrode and the upper electrode of each device and the air layer
interposed between the second ground line and the signal line have
substantially the same thickness.

[0060]According to an embodiment of the present invention, concurrent
formation of a variable device and a fixed passive device, each having a
nearly identical device structure, by using the same manufacturing
processes, can realize a variable device circuit having more stable,
lower-loss circuit characteristics with lower manufacturing cost, in
comparison with cases in which a variable device and a fixed passive
device, each having mutually different device structures, are formed
independently of each other by using different manufacturing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 is a perspective view showing an example of a variable device
circuit according to Embodiment 1 of the present invention;

[0063]FIG. 3 is an equivalent circuit diagram of the radio-frequency
variable component shown in FIG. 1;

[0064]FIGS. 4 to 11 show longitudinal sectional views each taken along the
lines A-A', B-B' and C-C' in FIG. 1, illustrating an example of a method
for manufacturing radio-frequency variable components;

[0065]FIG. 12 is a sectional view showing film nonuniformity of the
dielectric film at a step portion of the fixed capacitor;

[0066]FIG. 13 is a perspective view showing an example of a variable
device circuit according to Embodiment 2 of the invention;

[0067]FIG. 14 is a longitudinal sectional view taken along the line D-D'
in FIG. 13; and

[0068]FIG. 15 is a longitudinal sectional view taken along the line E-E'
in FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069]This application is based on an application No. 2007-99104 filed on
Apr. 5, 2007 in Japan, the disclosure of which is incorporated herein by
reference.

[0070]Hereinafter, preferred embodiments will be described with reference
to drawings.

[0071]Hereinbelow, embodiments of the present invention will be described
with reference to the accompanying drawings. Throughout the following
description of respective embodiments, similar components are designated
by the same reference numerals.

Embodiment 1

[0072]FIG. 1 is a perspective view showing an example of a variable device
circuit according to Embodiment 1 of the invention. FIG. 2 shows
longitudinal sectional views each taken along lines A-A', B-B' and C-C'
in FIG. 1. Although a radio-frequency variable component is described
below, the present invention is applicable to various types of
high-frequency circuits and low-frequency circuits, for example, circuits
in which variable devices and fixed passive devices are integrated on one
and the same substrate, and circuits in which variable devices, fixed
passive devices and common semiconductor active devices are integrated on
one and the same substrate.

[0073]The radio-frequency variable component includes a movable switch
device 21, a fixed capacitor device 22, a variable inductor device 23,
and a variable capacitor device 24, all of which are formed on the top
face of a substrate 1. For easier understanding, in this case, described
is an example in which each device is mounted one by one on the substrate
1. However, plural devices of each type may be mounted on the substrate
1, and common semiconductor active devices, such as transistors and
semiconductor integrated circuits, also may be mounted together therewith
on the substrate 1.

[0074]The substrate 1 is formed of semiconductor material such as silicon
and GsAs, or dielectric material such as glass, alumina and resin, or the
like. On the top face of the substrate 1, an insulating film 2 made of
oxide or other electric insulating material is formed throughout.

[0075]On the insulating film 2, a first conductive layer having a
predetermined pattern is formed. In this case, exemplified is the first
conductive layer formed of stacked layers of a first metal layer 3 and a
second metal layer 4 on the substrate side. Alternatively, the first
conductive layer may be formed of a single metal layer, or three or more
metal layers.

[0076]The first conductive layer, as shown in FIG. 2, can function as a
lower electrode 13 of the movable switch device 21, as a lower electrode
13 of the fixed capacitor device 22, as a lower electrode 13 of the
variable inductor device 23, and as a lower electrode 13 of the variable
capacitor device 24, and moreover as a wiring pattern for electrically
connecting the respective devices.

[0077]A second conductive layer having a predetermined pattern is formed
above the first conductive layer. In this case, exemplified is the second
conductive layer formed of stacked layers of a seed metal layer 8 and a
third metal layer 10 on the substrate side. Alternatively, the second
conductive layer may be formed of a single metal layer, or three or more
metal layers.

[0078]The second conductive layer, as shown in FIG. 2, can function as an
upper electrode of the movable switch device 21, as an upper electrode of
the fixed capacitor device 22, as an upper electrode of the variable
inductor device 23, and as an upper electrode of the variable capacitor
device 24, and moreover as a wiring pattern for electrically connecting
the respective devices.

[0079]Next, construction and operation of each device is explained below.

[0080]In the movable switch device 21, an air layer is formed between the
lower electrode 13 and the upper electrode, and a dielectric layer 5 is
formed on the lower electrode 13. The upper electrode has a cantilever
structure with one end being fixed to a particular wiring pattern and the
other end being swingable, and a protrusive contact 11 is formed on the
backside of the swingable tip end. Another wiring pattern is provided on
the substrate 1 so as to face to the contact 11. A bias line for driving
the device extends out from the lower electrode 13.

[0081]When a driving voltage is applied between the lower electrode 13 and
the upper electrode through the bias line, the tip end of the upper
electrode is flexurally deformed toward the substrate 1 by action of
electrostatic force, so that the contact 11 makes contact with the
opposed wiring pattern, resulting in a conducting state. In this
situation, the dielectric layer 5 that covers the lower electrode 13
functions as an anti-stiction film between the lower electrode 13 and the
upper electrode.

[0082]When application of the driving voltage is stopped, the contact 11
is returned away from the wiring by elastic force of the cantilever. In
this way, opening and closing of the contact 11 can be controlled
depending on whether the driving voltage is applied or not.

[0083]In the fixed capacitor device 22, the dielectric layer 5 is
interposed between the lower electrode 13 and the upper electrode, and
further an air layer 12 is interposed between electrodes so as to adjoin
the dielectric layer 5. The capacitance of the fixed capacitor device 22
can be defined by an area of the electrode, an inter-electrode distance
and a dielectric constant of the inter-electrode medium, and electric
charge is stored in response to a voltage applied between the electrodes.

[0084]In the variable inductor device 23, an air layer is formed between
the lower electrode 13 and the upper electrode, and a dielectric layer 5
is formed on the lower electrode 13. The upper electrode has a doubly
clamped beam structure with both ends being fixed to particular wiring
patterns and a central portion being swingable. A meander type or
spiral-type of inductor is formed at the central portion. It is noted
that a meander-type one is illustrated by way of an example in FIG. 1. A
bias line for driving the device extends out from the lower electrode 13.

[0085]When a driving voltage is applied between the lower electrode 13 and
the upper electrode through the bias line, the central portion of the
upper electrode is flexurally deformed toward the substrate 1 by action
of electrostatic force, so that the self inductance can be changed. In
this situation, the dielectric layer 5 that covers the lower electrode 13
functions as an anti-stiction film between the lower electrode 13 and the
upper electrode.

[0086]When application of the driving voltage is stopped, the upper
electrode is restored to its original configuration by elastic force of
the doubly clamped beam, where the self inductance returns to an initial
value. In this way, the self inductance can be controlled depending on
the magnitude of the driving voltage.

[0087]In the variable capacitor device 24, an air layer is formed between
the lower electrode 13 and the upper electrode, and a dielectric layer 5
is formed on the lower electrode 13. The upper electrode has a doubly
clamped beam structure with both ends being fixed to particular wiring
patterns and a central portion being swingable. A bias line for driving
the device extends out from the lower electrode 13.

[0088]When a driving voltage is applied to between the lower electrode 13
and the upper electrode through the bias line, the central portion of the
upper electrode is flexurally deformed toward the substrate 1 by action
of electrostatic force, so that the capacitance can be changed. In this
situation, the dielectric layer 5 that covers the lower electrode 13
functions as an anti-stiction film between the lower electrode 13 and the
upper electrode. When application of the driving voltage is stopped, the
upper electrode is restored to its original configuration by elastic
force of the doubly clamped beam, where the capacitance returns to an
initial value. In this way, the capacitance can be controlled depending
on the magnitude of the driving voltage.

[0089]FIG. 3 is an equivalent circuit diagram of the radio-frequency
variable component shown in FIG. 1. A series circuit of the movable
switch device 21 and the fixed capacitor device 22 is connected between a
node Na and a node Nb, and the variable capacitor device 24 is connected
in parallel to the series circuit. The variable inductor device 23 is
connected between the node Nb and a node Nc. Also, although not shown in
FIG. 1, a series circuit of a movable switch device 21a and a fixed
capacitor device 22a as well as a series circuit of a movable switch
device 21b and a fixed capacitor device 22b are connected between the
node Na and the node Nb.

[0090]With regard to the operation, when the movable switch devices 21,
21a and 21b are turned off, a series circuit of the variable capacitor
device 24 and the variable inductor device 23 is built up. The
capacitance of the variable capacitor device 24 can be continuously
controlled by the driving voltage of the bias line. The inductance of the
variable inductor device 23 can be continuously controlled by the driving
voltage of the other bias line. Accordingly, the resonance frequency of
the series circuits can be continuously controlled.

[0091]Next, when the movable switch device 21 is turned on, the
capacitance of the fixed capacitor device 22 is added so that the
combined capacitance increases stepwise. Next, when the movable switch
device 21a is turned on, the capacitance of the fixed capacitor device
22a is added so that the combined capacitance increases stepwise. Next,
when the movable switch device 21b is turned on, the capacitance of the
fixed capacitor device 22b is added so that the combined capacitance
increases stepwise. Accordingly, the resonance frequency of the series
circuits can be controlled stepwise by selective opening and closing of
the movable switch devices 21, 21a and 21b. In general, with use of N
movable switch devices, stepwise control of the resonance frequency is
implementable in 2N combinations.

[0092]Consequently, there can be realized a variable filter which can be
tuned with high accuracy over a wide range of resonance frequency by
stepwise control using a plurality of movable switch devices and by fine
adjustment using variable capacitor devices and variable inductor
devices.

[0093]In this case, a case in which a plurality of fixed capacitor devices
are selected by selective operation of a plurality of movable switch
devices has been shown by way of an example. Alternatively, selection of
a plurality of fixed inductor devices, selection of variable capacitor
devices and/or selection of a plurality of variable inductor devices can
be implemented by selective operation of a plurality of movable switch
devices.

[0094]Now, a method for manufacturing the radio-frequency variable
component shown in FIG. 1 is explained. First, as shown in FIG. 4, an
electrically insulating material, e.g., about 1 μm thick oxide, is
deposited on the top face of the substrate 1 by thermal oxidation process
or the like to form an insulating film 2.

[0095]Next, for a first conductive layer on the insulating film 2, an
electrically conductive material showing a relatively high resistance,
such as titanium, is deposited by sputtering or the like to form a first
metal layer 3. Subsequently, on the first metal layer 3, an electrically
conductive material showing a relatively low resistance, such as gold, is
deposited by sputtering or the like to form a second metal layer 4.

[0096]The first metal layer 3 is used to improve the degree of adhesion
between the second metal layer 4 and the insulating film 2. Further, as
described later, it is preferable that when the bias line is formed only
of the first metal layer 3, an electrically conductive material and
cross-sectional dimensions showing a relatively high resistance are
adopted for the first metal layer 3 so that a high-resistance bias line
can be obtained.

[0097]For the second metal layer 4, it is preferable to adopt such a
material and cross-sectional dimensions that the degree of adhesion with
the later-described seed layer 8 for the second conductive layer can be
ensured.

[0098]Next, as shown in FIG. 5, resist patterning and dry etching
processes are carried out on the first metal layer 3 and the second metal
layer 4 to form the lower electrode of the movable switch device 21, the
lower electrode of the fixed capacitor device 22, the lower electrode of
the variable inductor device 23, and the lower electrode of the variable
capacitor device 24 as well as the wiring patterns for electrical
connection of the respective devices.

[0099]Next, as shown in FIG. 6, in order to form a high-resistance bias
line, resist patterning and wet etching processes are carried out on the
second metal layer 4 to partly remove the second metal layer 4, so that
the bias line is formed only of the first metal layer 3.

[0100]Next, as shown in FIG. 7, a dielectric material, e.g., about 1 μm
thick nitride, is deposited on the first metal layer 3 or the second
metal layer 4 by sputtering or the like. Thereafter, resist patterning
and dry etching processes are carried out to form a dielectric layer 5 on
the lower electrodes of the movable switch device 21, the fixed capacitor
device 22, the variable inductor device 23 and the variable capacitor
device 24 as well as on the bias line, respectively. In this case, the
thicknesses of the dielectric layer 5 lying on the lower electrodes of
the respective devices and the bias line may be different from one
another.

[0101]Next, as shown in FIG. 8, a metal material, e.g., about 1.5 μm
thick nickel, is deposited by sputtering or the like. Thereafter, resist
patterning and dry etching processes are carried out to form a
sacrificial layer 6 on the lower electrodes of the movable switch device
21, the fixed capacitor device 22, the variable inductor device 23 and
the variable capacitor device 24 as well as on the bias line,
respectively, so as to correspond to the configuration and placement of
the air layer of each device such as shown in FIG. 2.

[0102]In particular, in the movable switch device 21, the sacrificial
layer 6 defines the configuration of the lower face of the upper
electrode having a cantilever structure. In the fixed capacitor device
22, the sacrificial layer 6 is formed so as to extend from the top face
of the substrate 1 beyond the lower electrode and reach part of the
dielectric layer 5. In the variable inductor device 23 and the variable
capacitor device 24, the sacrificial layer 6 defines the configuration of
the lower face of the upper electrode having a doubly clamped beam
structure.

[0103]Next, as shown in FIG. 9, resist patterning and dry etching
processes are carried out on the sacrificial layer 6 to form a dimpled
shape 6a for shaping a contact 11 of the movable switch device 21.

[0104]Next, as shown in FIG. 10, on the dielectric layer 5 of the fixed
capacitor device 22, a conductor material, such as Cr (chromium), is
deposited by sputtering or the like to improve the degree of adhesion
between the dielectric layer 5 and the seed layer 8. Thereafter, resist
patterning and wet etching processes are carried out to form an adhesion
layer 7.

[0105]Next, as shown in FIG. 11, a conductor material, such as gold, is
deposited by sputtering or the like to form a seed layer 8. The seed
layer 8 is used for the succeeding step of thickening a third metal layer
10 using electroplating or the like.

[0106]Next, patterning of resist 9 is carried out on the seed layer 8, and
thereafter a thick-film plated layer having a thickness of several
microns is formed using, e.g., electroplating or the like, to form a
third metal layer 10. The seed layer 8 and the third metal layer 10
constitute a second conductive layer acting as the upper electrode of
each device.

[0107]Next, the resist 9 is removed and thereafter dry etching is carried
out throughout the substrate 1 to remove the seed layer 8 under the
resist 9. Subsequently, the sacrificial layer 6 is removed by wet etching
to obtain a hollow-structured device having an air layer corresponding to
the configuration of the sacrificial layer 6, as shown in FIG. 2.

[0108]In this embodiment, in the fixed capacitor device 22, not only the
dielectric layer 5 but also the air layer 12 formed by removal of the
sacrificial layer 6 are provided together between the lower electrode and
the upper electrode. In the fixed capacitor device 22, in the case where
the dielectric layer 5 is deposited by sputtering process or the like,
since the material is deposited vertically on the substrate, the
deposition rate at the step portion (directed parallel to the substrate)
due to the thickness of the lower electrode is slower than that in the
vertical direction with respect to the substrate, so that film
nonuniformities of the dielectric layer 5 may occur as shown in FIG. 12,
causing a possibility of electrical short circuits between the lower
electrode and the upper electrode. As a countermeasure therefor, making
the sacrificial layer 6 thicker than the dielectric layer 5 allows the
air layer 12 to prevent such short circuits.

[0109]Further, additionally providing the air layer 12, which is of the
lowest dielectric constant, in the fixed capacitor device 22 makes it
possible to suppress a fringing effect between the lower electrode and
the upper electrode. This produces an advantage that a capacitance as
designed can be obtained.

[0110]Also, the dielectric layer 5 provided above the lower electrodes 13
of the movable switch device 21, the variable inductor device 23 and the
variable capacitor device 24, respectively, functions as an anti-stiction
film between the lower electrode 13 and the upper electrode.

[0111]Further, since the dielectric layer 5 is so formed as to cover the
entire lower electrode of each device to prevent the lower electrode from
being etched in the etching process of the seed layer 8 after the
formation of the third metal layer, the fixed capacitor 22 and the
variable capacitor 24 both of which are electrically stable and low in
loss can be obtained.

[0112]Also in this embodiment, since the upper electrode of each device is
thickened by the electroplating process using the seed layer 8, low-loss
high frequency characteristics can be obtained by lowering the resistance
of the upper electrode. This also further enhances the mechanical
strength of the devices, thereby improving the device reliability.

[0113]Moreover, since the bias lines for driving the movable switch device
21, the variable inductor device 23 and the variable capacitor device 24
are formed of the first metal layer 3 made of an electrically conductive
material, such as titanium, showing a relatively high resistance, hence,
the bias lines can be higher in resistance.

[0114]Furthermore, in the manufacturing process of the radio-frequency
variable component, forming the dielectric layer 5 and the sacrificial
layer 6 also on the bias lines as shown in the C-C' sectional views of
FIGS. 7 to 11 allows the bias lines to be securely prevented from being
etched in the etching process of the dielectric layer 5 as well as in the
etching process of the seed layer 8, hence, the bias lines can be higher
in resistance by thinning the bias lines.

[0115]As a result, the higher resistance of the bias line can suppress
leakage of high-frequency waves flowing through signal lines to the lower
electrode 13 of each device, thereby preventing an increase in insertion
loss of the high-frequency waves.

[0116]Also, the formation of the dimpled shape 6a for the contact 11 of
the switch 21 is processed not simultaneously with the deposition of the
sacrificial layer 6 but after the deposition and patterning of the
sacrificial layer 6, hence, contaminants such as resist are less
accumulated at the bottom of the dimpled shape 6a. Thus, a clean surface
of the contact 11 of the switch 21 can be obtained, resulting in low-loss
high frequency characteristics.

[0117]As described above, concurrent formation of the movable switch
device 21, the fixed capacitor device 22, the variable inductor device 23
and the variable capacitor device 24, each having a nearly identical
hollow structure, by using the same manufacturing processes, can realize
a variable device circuit having more stable, lower-loss circuit
characteristics with lower manufacturing cost.

Embodiment 2

[0118]FIG. 13 is a perspective view showing an example of a variable
device circuit according to Embodiment 2 of the invention. FIG. 14 is a
longitudinal sectional view taken along the line D-D' in FIG. 13. FIG. 15
is a longitudinal sectional view taken along the line E-E' in FIG. 13.
This embodiment will be described below by way of an example of such a
radio-frequency variable component as in Embodiment 1.

[0119]For easier understanding, in this case, described is an example in
which a series circuit of the movable switch device 21 and the fixed
capacitor device 22 is formed on the top face of the substrate 1.
However, the movable switch device 21, the fixed capacitor device 22, the
variable inductor device 23, the variable capacitor device 24 and various
active devices may be mounted each in a single or plural form on the
substrate 1 as in Embodiment 1.

[0120]Also, since the movable switch device 21 and the fixed capacitor
device 22 are similar in construction and operation to those of
Embodiment 1, and so their duplicated description is omitted.

[0121]On the top face of the substrate 1, provided is a coplanar
transmission line including a signal line in which a series circuit of
the movable switch device 21 and the fixed capacitor device 22 is
inserted, and ground lines 14 located on both sides of the signal line.

[0122]The signal line and the ground lines 14 can be concurrently formed
with a desired pattern during the process of forming the first conductive
layer (first metal layer 3, second metal layer 4, etc.) and/or the second
conductive layer (seed layer 8, third metal layer 10, etc.) in the method
for manufacturing the radio-frequency variable component as shown in
FIGS. 4 to 11.

[0123]The bias line extending from the lower electrode of the movable
switch device 21 is connected to a pad (not shown) by intersecting the
ground line 14 with an interposed air layer. A dielectric layer 5 is
formed on the bias line as shown in FIG. 7. Further, a sacrificial layer
6 is formed as shown in FIG. 8, thereafter a seed layer 8 is formed on
the sacrificial layer 6 as shown in FIG. 11, and then a third metal layer
10 is formed by electroplating or the like so as to form a bridging
portion of the ground lines 14. Subsequently, the sacrificial layer 6 is
removed by wet etching, so that an air layer 12 can be formed below the
bridging portion as shown in FIG. 14. The interposition of the air layer
can securely prevent electrical short circuits between the bias line and
the ground lines.

[0124]In this case, described above is an example in which the bias line
is formed by patterning the first conductive layer and the bridging
portion of the ground line 14 is formed by patterning the second
conductive layer. Alternatively, the ground line 14 may be formed by
patterning the first conductive layer and a bridging portion of the bias
line may be formed by patterning the second conductive layer.

[0125]Further, a second ground line 14a is provided for electrically
connecting the ground lines 14 on both sides of the signal line to each
other.

[0126]The second ground line 14a can be concurrently formed simultaneously
with a desired pattern during the process of forming the first conductive
layer (first metal layer 3, second metal layer 4, etc.) and/or the second
conductive layer (seed layer 8, third metal layer 10, etc.) in the method
for manufacturing the radio-frequency variable component as shown in
FIGS. 4 to 11.

[0127]The second ground line 14a intersects the signal line with an
interposed air layer. A sacrificial layer 6 is formed on the second
ground line 14a as shown in FIG. 8, thereafter a seed layer 8 is formed
on the sacrificial layer 6 as shown in FIG. 11, and then a third metal
layer 10 is formed by electroplating or the like so as to form a bridging
portion of the signal line. Subsequently, the sacrificial layer 6 is
removed by wet etching, so that an air layer 12 can be formed below the
bridging portion as shown in FIG. 15. The interposition of the air layer
can securely prevent electrical short circuits between the second ground
line 14a and the signal line.

[0128]In this case, described above is an example in which the second
ground line 14a is formed by patterning the first conductive layer and
the bridging portion of the signal line is formed by patterning the
second conductive layer. Alternatively, the signal line may be formed by
patterning the first conductive layer and a bridging portion of the
second ground line 14a may be formed by patterning the second conductive
layer.

[0129]Further, described above is an example in which the second ground
line 14a is formed below the bridging portion of the signal line.
Alternatively, the second ground line 14a may also be fabricated so as to
run below the variable capacitor device 24.

[0130]In this case, the air layer 12 interposed between the lower
electrode and the upper electrode of such passive devices as the movable
switch device 21 and the fixed capacitor device 22, the air layer 12
interposed between the bias line and the ground lines, and the air layer
12 interposed between the second ground line and the signal line are
formed by removal of the sacrificial layer 6. Although the sacrificial
layer 6 may be varied in thickness depending on where the sacrificial
layer 6 is left, yet forming the respective sacrificial layers 6 to the
same thickness can eliminate constraints on the patterning accuracy of
the sacrificial layer 6, so that the respective components can be
fabricated in a distance closer to one another, thereby enhancing the
integration density. Further, the air layers corresponding to those
sacrificial layers 6 are substantially equal in thickness to one another,
so that stable electrical characteristics can be achieved.

[0131]In the description given above, described is an example in which the
movable switch device 21, the fixed capacitor device 22, the variable
inductor device 23, and the variable capacitor device 24 are formed on
one side of the substrate 1. In addition to this, the fixed inductor
device may also be formed together.

[0132]Also, described above is an example in which a meander-type of
inductor is used for the variable inductor device 23. However, without
limiting to this, a spiral-type of inductor may be used in the present
invention.

[0133]Also, described above is an example in which a bridge-type of
movable portion is used for the variable capacitor device 24. However,
without limiting to this, a cantilever-type of movable portion may be
used in the present invention.

[0134]Also, described above is an example in which a cantilever-type of
metal-contact switch is used for the movable switch device 21. However,
without limiting to this, other mechanical-type of switch having a
mechanical drive mechanism may be used in the present invention.

[0135]Also, described above is an example in which titanium is used for
the first metal layer 3, and an electrically conductive material made of
gold is used for the second metal layer 4, the seed layer 8 and the third
metal layer 10. However, without limiting to this, other metal materials,
such as silver or copper, may be used to form a conductor with a
predetermined configuration by carrying out patterning and etching
processes based on photolithography techniques in the present invention.

[0136]Also, described above is an example in which a dielectric material
made of nitride is used for the dielectric layer 5. However, without
limiting to this, another dielectric material, such as oxide, may be used
to form a dielectric with a predetermined configuration by carrying out
patterning and etching processes based on photolithography techniques in
the present invention.

[0137]Also, described above is an example in which nickel is used for the
sacrificial layer 6. However, without limiting to this, any other
materials may be used to form a sacrificial layer with a predetermined
configuration by carrying out patterning and etching processes based on
photolithography techniques, as long as it can be removed using etching
at the final step.

[0138]Also, described above is an example in which a silicon substrate is
used for the substrate 1. However, without limiting to this, dielectric
substrate, such as glass substrate, alumina substrate, resin substrate,
or semiconductor substrate, such as GaAs substrate, may be used in the
present invention.

[0139]Although the present invention has been fully described in
connection with the preferred embodiments thereof and the accompanying
drawings, it is to be noted that various changes and modifications are
apparent to those skilled in the art. Such changes and modifications are
to be understood as included within the scope of the present invention as
defined by the appended claims unless they depart therefrom.